In the conventional auger-type extruder, potentially fusible plastic material or resin, commonly in pellet, granule, powder or other particulate form, is introduced into the feed section of the extruder and conveyed as a substantially undisturbed body except for compaction. As it is carried toward the transition or melting zone while still in a particulate condition, the temperature thereof increases as heat transfers from the barrel into the outer surface of the body. Friction of the work body with the screw and the barrel also heats the body. However, the rate of temperature rise in the resinous body is primarily dependent upon the rate of heat transfer occurring at the interface of the inner barrel and the body or, more specifically, on the temperature difference between the barrel and the compacting plastic particles.
Heating of the body while particulate is inherently poor because of the inability to bring the particles at various depths within the body into continuous contact with the inner barrel surface. Heating of the resin becomes even more inefficient as the resin progresses lengthwise of the barrel because of the lessening in temperature difference between the barrel and the interfacial particles. The resin, while in particulate form, has quite low heat conductivity, and in demanding peak output from an extruder by operating at somewhat hazardly high barrel temperatures, the resin adjacent to the barrel may overheat while the interior of the body remains considerably cooler. Once the particulate material has reached fusion temperature, the work load may be subject to somewhat different heating principles and different screw structure, e.g., that described in U.S. Pat. No. 3,701,512 or U.S. Pat. No. 3,698,541.
It has been discovered that the heat exchange rate between the barrel and the granular resin body can be increased substantially throughout the apparatus through which it passes, such as a section of an extruder upstream from its melting section by causing continuous movement of a layer of particles adjacent the interface of granular body with the barrel transversely of the channel so as to circulate the particles along a transverse cross section of the body and thereby force into successive contact with the barrel the cooler portion of the body normally remaining radially inwardly of the channel. This principle of operation causes not only greater heat pickup by the resin at its interface with the barrel but enables the heated particles to move inwardly of the channel to dissipate heat into other portions of the particulate body.
An object of the invention is to provide an apparatus having special facility for heating granular materials, and when incorporated integrally within the structure of an extruder, to substantially increase the throughput thereof, and greatly increase its capacity as measured in pounds of extruded material per horsepower per hour over conventional machines of like size.